The present invention relates to a drive system for a program shutter of a camera which utilizes a ultrasonic motor.
Many automatic exposure (AE) compact cameras incorporated with program shutters are widely spreaded. Such a program shutter controls shutter speed and aperture according to an exposure value so as to make a programmed exposure.
In recent years, the program shutter, which consists of a plurality of shutter blades, has been developed with accurate exposure control obtained by a stepping motor. Using the stepping motor contributes to simplifying the structure of program shutter and its associated elements.
Stepping motors should have a driving torque sufficient for a steady action of shutter blades. The conventional stepping motors, however, cause unsteady shutter blades action. The stepping motor rotates stepwise through a fixed angle, so that the speed of program shutter is restricted depending upon the stepwise rotatable angle of the stepping motor. This leads to limitations of designing the program shutter.
Furthermore, the stepwise rotation of the stepping motor is controlled independently from the actual actions of the shutter blades or the actual rotation of the stepping motor, so as to decrease the accuracy of programmed exposure control.